Carbothermal synthesis of cubic aluminium oxynitride (ALON) have been performed in a graphite furnace at different temperatures (1700–1860 °C) and pressures (0.1–10 MPa). The products were characterized by XRD, SEM and elemental analyses, and the overall compositions were determined by reoxidation in air. The control of excess nitridation was a crucial task in the preparation of ALON. A two-step procedure, first annealing at about 1550 °C then at about 1800 °C, was found to be effective to this end. CO additions to the N2 gas stimulated ALON formation, but a pressure increase resulted in a lower nitridation rate. The particle size and morphology of ALON depended on raw material type and synthesis parameters such as temperature program, pressure, CO content in the N2 gas, and sample compartment type. Thermodynamic calculations in the Al-O-C-N system have been performed in order to explain the experimental results.

Carbothermal synthesis of Aluminium Oxynitride (ALON) powder: Influence of starting materials and synthesis parameters

Characterization of carbon-bonded alumina filters with active or reactive coatings in a steel casting simulator

A sessile-drop study was carried out on Al2O3-10 pct C refractory substrates in contact with molten iron to investigate possible chemical reactions in the system and to determine the influence of carbon and the role, if any, played by the presence of molten iron that can act both as a reducing agent and as a metallic solvent. These investigations were carried out at 1823 K (1550 °C) in argon atmosphere for times ranging between 15 minutes and 3 hours. We report the occurrence of chemical reactions in the Al2O3-10 pct C/Fe system, associated generation of CO gas, and carbon pickup by molten iron. Video images of the iron droplet started to show minor deviations after 30 minutes of contact followed by intense activity in the form of fine aluminum oxide whiskers emanating from the droplet and on the refractory substrate. The interfacial region also changed significantly over time, and the formation of small quantities of iron aluminide intermetallics was recorded after 30 minutes as a reaction product in the interfacial region. These chemical reactions also caused extensive penetration of molten iron into the refractory substrate. This study has shown that alumina cannot be treated as chemically inert at steelmaking temperatures when both carbon and molten iron are present simultaneously. These findings point to an additional reaction pathway during steelmaking that could have significant implications for refractory degradation and contamination of steel with reaction products.

Chemical Interactions of Alumina–Carbon Refractories with Molten Steel at 1823 K (1550 °C): Implications for Refractory Degradation and Steel Quality

In research conducted by the United States Bureau of Mines, the Sn-C and Al-Sn-C phase diagrams were determined over the temperature range of 1550 °C to 2300 °C by chemical anal-ysis of alloys saturated with carbon within sealed graphite crucibles. Carbon forms a dilute solution in tin described by log [at. pct C] = 2.9767-12,082.35/T, whereT is temperature in Kelvin. Isothermal sections for the ternary system were determined at intervals of 150 °C over the range of temperatures investigated. The univariant points on the 1700 °C, 1850 °C, and 2000 °C isotherms were determined by metallographic examination of rapidly cooled alloys to be about 34Al-66Sn-0.1C, 49.7Al-49.7Sn-0.5C, and 70Al-27Sn-2.8C, respectively, where all concentrations are atomic percent. Graphite and A14C3 (decomposition temperature 2156 °C) were the only solid phases observed at these temperatures. The excess partial Gibbs energy for dissolved C in liquid Al-Sn-C solutions in equilibrium with C, as calculated from the experi-mental solubilities, is G
 C
 e
 = -RT ln x = y2[176.860 - 55.42T - (224,200 - 110.84T)x] + (231,400 - 18.700T)z2 + yz[151,860 - 8.423T + (19,400 - 8.4867)z + (56,100 - 57.6577)yz - (39,800 - 40.904T)yz2], J/g · atom where R is the gas constant,T is the temperature in Kelvin, andx, y, and z are the atomic fractions of C, Al, and Sn, respectively. The equation also is a good approximation for liquid solutions in equilibrium with A14C3 within about 100 °C of the decomposition temperature.

Sn-C and Al-Sn-C phase diagrams and thermodynamic properties of C in the alloys: 1550 °C to 2300 °C

This paper is the first part of an investigation of carbothermal nitridation of alumina at elevated nitrogen pressure. The effect of some process parameters on nitridation rate was studied at different temperatures (1300–1700 °C) and pressures (0.1–5 MPa). The experiments were performed in a graphite furnace permitting a controlled nitrogen flow through the sample holder, and continuous measurement of CO in the outlet gas. The products were characterized by X-ray diffraction, BET and elemental analyses. The nitridation rate was found to be a function of the process parameters. For a suitable temperature and pressure, e.g. 1600 °C and 1 MPa, pure AIN was obtained after just 1 h, with very low residual oxygen content (0.4 wt%). At a high gas-flow rate (⩾5 I min−1), the optimum range of pressure was 0.5–1 MPa at 1550–1700 °C. The nitridation was hampered by even small amounts of CO added to the system. No intermediate phases formed under the experimental conditions studied. Thermodynamic calculations on the Al-O-C-N system were performed in a search for explanations of the results.

Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures

The carbothermic reduction of alumina was studied in the temperature range of 1700°C to 1850°C in the presence of either tin or copper as the metallic solvent. The total pressure in the smelting system was controlled at pressures between 0.08 and 0.20 atm. The overall reaction is Al2O3(s)+3C(s)=2Al+3CO(g). The rate of reduction of alumina was found to depend strongly on temperature, increasing by three orders of magnitude between 1700°C and 1850°C. Total pressure and activity of aluminum in the solvent bath also affected the rate of reduction. Changes in the alumina particle type and size, in the carbon type, in the carbon-to-oxygen ratio, and in the pellet size had little effect on the rate of reduction of alumina. The kinetics of reduction are shown to follow a pseudo-first order kinetic model.

Charles W. Finn

Papers

Physical chemistry of the carbothermic reduction of alumina in the presence of a metallic solvent: Part II. Measurements of kinetics of reaction